Image sensing devices, such as a charge-coupled device (CCD), are commonly found in such products as digital cameras, scanners, and video cameras. These image sensing devices have a very limited dynamic range when compared to traditional negative film products. A typical image sensing device has a dynamic range of about 5 stops. This means that the exposure for a typical scene must be determined with a fair amount of accuracy in order to avoid clipping the signal. In addition, oftentimes the scene has a very wide dynamic range as a result of multiple illuminants (e.g. frontlit and backlit portions of a scene). In the case of a wide dynamic range scene, choosing an appropriate exposure for the subject often necessitates clipping data in another part of the image. Thus, the inferior dynamic range of an image sensing device relative to silver halide media results in lower image quality for images obtained by an image sensing device.
An increase in the dynamic range of an image sensing device would allow images from digital cameras to be rebalanced to achieve a more pleasing rendition of the image. Also, increasing the dynamic range of an image sensing device would allow for more pleasing contrast improvements to the image, such as is described by Lee et al. in commonly assigned U.S. Pat. No. 5,012,333.
U.S. Pat. No. 6,040,858 (Ikeda) provides a complete description of the problem of the limited dynamic range of image sensing devices. In addition, Ikeda describes methods of extending the dynamic range of an image sensing device by utilizing multiple image signals, each with different responses to exposure. These multiple signals are combined by using thresholds, which determine which signal is of higher quality at each position in the image signal to form an image signal having extended dynamic range. Ikeda improves upon these methods by describing a method by which these thresholds are determined for each color.
However, these prior art methods, including Ikeda, require multiple image signals to form an image signal having extended dynamic range. Attaining such multiple signals can cause difficulty. For example, if the multiple image signals are not captured simultaneously, objects moving in the scene or motion of the camera may produce artifacts in an image signal having extended dynamic range. Additionally, if the image signals are captured simultaneously but on separate image capture devices, then a correspondence problem exists. Moreover, the additional hardware adds undesirable cost to the imaging system.
U.S. Pat. No. 5,051,770 to Cornuejols discloses a device, in which a flat screen modifies, on a pixel by pixel basis, light received by a sensor.
U.S. Published Patent Application 2003/0052976 describes a photographic device having photosites of different sensitivities, controlled by drive voltages. Different exposure times can be imparted to the photosites.
Thus, there exists a need to improve upon the method of the prior art in order to improve the dynamic range of an image sensing device. Specifically, there exists a need to produce an extended dynamic range image signal with a single image sensing device and a single image signal.